Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-MRAM). STT-MRAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction. A spin polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state.
For example, FIG. 1 depicts a conventional magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-MRAM. The conventional MTJ 10 typically resides on a substrate 12. A bottom contact 14 and top contact 22 may be used to drive current through the conventional MTJ 10. The conventional MTJ, uses conventional seed layer(s) (not shown), may include capping layers (not shown) and may include a conventional antiferromagnetic (AFM) layer (not shown). The conventional magnetic junction 10 includes a conventional pinned layer 16, a conventional tunneling barrier layer 18, and a conventional free layer 20. Also shown is top contact 22. Conventional contacts 14 and 22 are used in driving the current in a current-perpendicular-to-plane (CPP) direction, or along the z-axis as shown in FIG. 1. Typically, the conventional pinned layer 16 is closest to the substrate 12 of the layers 16, 18 and 20.
The conventional pinned layer 16 and the conventional free layer 20 are magnetic. The magnetization 17 of the conventional pinned layer 16 is fixed, or pinned, in a particular direction. Although depicted as a simple (single) layer, the conventional pinned layer 16 may include multiple layers. For example, the conventional pinned layer 16 may be a synthetic antiferromagnetic (SAF) layer including magnetic layers antiferromagnetically coupled through thin conductive layers, such as Ru. The conventional pinned layer 16 may also be another multilayer. The conventional free layer 20 has a changeable magnetization 21. Although depicted as a simple layer, the conventional free layer 20 may also include multiple layers. The conventional pinned layer 16 and free layer 20 may have their magnetizations 17 and 21, respectively oriented perpendicular to the plane of the layers.
In order to achieve perpendicular magnetic moments 17 and 21, various structures have been proposed. For example, the conventional pinned layer 16 may include a nonmagnetic coupling layers, such as a W or Ta layer, between a bulk perpendicular magnetic anisotropy layer (B-PMA) and an interfacial perpendicular magnetic anisotropy (I-PMA) layer. The B-PMA layer has a perpendicular magnetic anisotropy due to entire layer. Such a B-PMA layer may be a Co/Pt multilayer or alloy. The I-PMA layer has its perpendicular magnetic anisotropy dominated by phenomena at the interface. For example, the interfacial perpendicular magnetic anisotropy includes layers such as CoFeB and FeB. The I-PMA, B-PMA and coupling layers all, however, have drawbacks. The B-PMA layer may have a low spin polarization, adversely affecting the magnetoresistance of the conventional magnetic junction 10. The nonmagnetic insertion layer may diffuse during fabrication, which adversely affects the perpendicular magnetic anisotropy and magnetoresistance. The I-PMA layer may not have a sufficiently high perpendicular magnetic anisotropy to adequately pin the magnetic moment 17. Thus, the magnetic junction 10 may have a low magnetoresistance, poor stability of the pinned layer 16, and/or other drawbacks. Accordingly, what is needed is a method and system that may improve the performance of the spin transfer torque based memories. The method and system described herein address such a need.